U.S. patent number 5,597,873 [Application Number 08/450,267] was granted by the patent office on 1997-01-28 for superabsorbent polymers and products therefrom.
This patent grant is currently assigned to Hoechst Celanese Corporation. Invention is credited to Douglas R. Chambers, William G-J Chiang, Guy T. Woodrum.
United States Patent |
5,597,873 |
Chambers , et al. |
January 28, 1997 |
Superabsorbent polymers and products therefrom
Abstract
A surface crosslinked superabsorbent polymer composition, a
process for its preparation and articles made therefrom wherein
said superabsorbent composition is prepared by surface crosslinking
the polymerization product of a carboxyl or carboxylate group
containing monomer with a crosslinker solution comprising water,
C.sub.3 -C.sub.6 diol and a crosslinking compound having at least
two functional reactive groups with the carboxyl or carboxylate
groups of said polymerization product.
Inventors: |
Chambers; Douglas R.
(Chesapeake, VA), Chiang; William G-J (Virginia Beach,
VA), Woodrum; Guy T. (Chesapeake, VA) |
Assignee: |
Hoechst Celanese Corporation
(Somerville, NJ)
|
Family
ID: |
22847810 |
Appl.
No.: |
08/450,267 |
Filed: |
May 25, 1995 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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226160 |
Apr 11, 1994 |
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Current U.S.
Class: |
525/330.1 |
Current CPC
Class: |
C08F
8/14 (20130101); C08F 2810/20 (20130101); C08F
2810/30 (20130101) |
Current International
Class: |
C08F
8/00 (20060101); C08F 8/14 (20060101); C08F
008/14 () |
Field of
Search: |
;525/330.1 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Cain; Edward J.
Attorney, Agent or Firm: Crall; Hugh C.
Parent Case Text
This is a continuation of application(s) Ser. No. 08/226,160 filed
on Apr. 11, 1994 now abandoned.
Claims
We claim:
1. A superabsorbent polymer having a 0.6 psi AUL value of at least
25 g/g and a centrifuge retention value of at least 35 g/g wherein
said superabsorbent polymer is the product of a process which
comprises dispersing an aqueous solution of C.sub.3 -C.sub.6 diol
and a crosslinking compound having at least two reactive,
functional groups onto a crosslinked, partially neutralized,
carboxyl or carboxylate group base polymer having a 0.3 psi AUL
value of 15 g/g or less and heating said dispersion to crosslink
said polymer wherein said aqueous solution has a surface tension
not greater than about 55 dynes per cm.
2. A superabsorbent polymer according to claim 1 wherein said base
polymer is a partially neutralized polyacrylic acid polymer.
3. A superabsorbent polymer according to claim 1 wherein said base
polymer has a centrifuge retention value of at least 40 g/g.
4. A superabsorbent polymer according to claim 1 wherein said base
polymer is a copolymer of acrylic acid and a water soluble hydroxy
group polymer.
5. A superabsorbent polymer according to claim 1 wherein said
C.sub.3 -C.sub.6 diol is propylene glycol and the surface tension
of said aqueous solution is not greater than about 50 dynes per
cm.
6. A superabsorbent polymer according to claim 1 wherein said
crosslinking compound is'selected from the group consisting of
diglycidyl ether compounds, polyamide-epichlorohydrin adducts,
polyamine epichlorohydrin adducts and amine polymer-epichlorohydrin
adducts.
7. A superabsorbent polymer according to claim 1 wherein said
crosslinking compound is an amine polymer-epichlorohydrin adduct
wherein at least 50 mole percent of the reactive groups of said
adduct are the azetidinium group.
8. A superabsorbent polymer according to claim 1 wherein said
crosslinking compound is an amine polymer-epichlorohydrin adduct
wherein about 90 mole percent of the reactive groups of said adduct
are the azetidinium group.
9. A process for preparing a superabsorbent polymer which comprises
dispersing an aqueous solution of a C.sub.3 -C.sub.6 diol and a
crosslinking compound onto the surface of a crosslinked, partially
neutralized, carboxyl or carboxylate base polymer having a 0.3 psi
AUL value of 15 g/g or less and heating the mixture to crosslink
said polymer wherein said aqueous solution has a surface tension
not greater than about 55 dynes per cm and wherein said aqueous
solution comprises from about 0.5 to about 3.5 weight percent water
and from about 1.0 to about 2.5 weight percent of said C.sub.3
-C.sub.6 diol.
10. A process according to claim 9 wherein said based polymer is a
partially neutralized polyacrylic acid.
11. A process according to claim 9 wherein said base polymer has a
centrifuge retention value of at least 40 g/g.
12. A process according to claim 9 wherein a superabsorbent polymer
according to claim 8 wherein said base polymer is a copolymer of
acrylic acid and a water soluble hydroxy group polymer.
13. A process according to claim 9 wherein said C.sub.3 -C.sub.6
diol is propylene glycol and the surface tension of said aqueous
solution is not greater than about 50 dynes per cm.
14. A process according to claim 9 wherein said crosslinking
compound is selected from the group consisting of diglycidyl ether
compounds, polyamide-epichlorohydrin adducts, polyamine
epichlorohydrin adducts and amine polymer-epichlorohydrin
adducts.
15. A process according to claim 9 wherein said crosslinking
compound is an amine polymer-epichlorohydrin adduct wherein at
least 50 mole percent of the reactive groups of said adduct are the
azetidinium group.
16. A process according to claim 9 wherein said crosslinking
compound is an amine polymer-epichlorohydrin adduct wherein about
90 mole percent of the reactive groups of said adduct are the
azetidinium group.
17. An absorbent article comprising from about 5 to about 90
percent by weight of a superabsorbent polymer composition according
to claim 1 and about 10 to about 95 percent by weight of a
hydrophilic fiber.
18. An absorbent article comprising from about 30 to about 90
percent by weight of a superabsorbent polymer composition according
to claim 1 and about 10 to about 70 percent by weight of a
hydrophilic fiber.
19. An absorbent article comprising from about 30 to about 70
percent by weight of a superabsorbent polymer composition according
to claim 1 and about 30 to about 70 percent by weight of a
hydrophilic fiber.
20. A superabsorbent polymer according to claim 1 wherein said
crosslinking compound is diethylene glycol diglicidyl ether.
Description
BACKGROUND OF THE INVENTION
Technical Field
This invention is directed to improved aqueous fluid absorbent
polymers, a process for their manufacture and absorbent articles
made therefrom. In particular, the invention is directed to
superabsorbent polymers having improved fluid absorption
properties, which provide superior performance when incorporated
into absorbent articles.
Background
The present invention is directed to water-insoluble, crosslinked,
high molecular weight polymers capable of absorbing and retaining
large quantities of aqueous fluids. These polymers are well known
in the art by various names such as superabsorbent polymers,
hydrogels, hydrocolloids, water absorbent hydrophilic polymers,
etc. For the purpose of this description the term, "superabsorbent
polymer(s)" is used to describe such materials.
Exemplary superabsorbent polymers are crosslinked, partially
neutralized polyacrylic acid (see U.S. Pat. No. 4,654,039), a
crosslinked, partially neutralized starch-acrylic acid graft
polymer (U.S. Pat. No. 4,076,663), a crosslinked, partially
neutralized copolymer of isobutylene and maleic anhydride (U.S.
Pat. No. 4,389,513), a saponification product of vinyl
acetate-acrylic acid copolymer (U.S. Pat. No. 4,124,748), a
hydrolyzate of acrylamide polymer or acrylamide copolymer (U.S.
Pat. No. 3,959,569) or a hydrolyzate of an acrylonitrile copolymer
(U.S. Pat. No. 3,935,099). The teachings of the above patents are
hereby incorporated by reference.
Superabsorbent polymers find use in many fluid absorption
applications with the primary use being in the field of personal
care products such as diapers, sanitary napkins, adult incontinent
products, absorption pads for medical uses, etc. The largest market
for superabsorbent polymers is found in disposable diapers for
infants; see e.g. U.S. Pat. Nos. 3,669,103; 3,670,731 or
4,654,039.
Superabsorbent polymers are prepared by polymerizing an
ethylenically unsaturated monomer or alkali metal salt of such
monomers or mixtures thereof and crosslinking the polymer either
during and/or after polymerization. Preferably the ethylenically
unsaturated monomer is water soluble. However, monomers which
become water soluble by hydrolysis may be employed.
Exemplary water soluble monomers are those containing carboxyl
groups, carboxylic acid anhydride groups, carboxylic acid salt
groups, sulfonic acid groups, sulfonic acid salt groups, hydroxyl
groups, amide groups, amino groups and quaternary ammonium salt
groups. An extensive listing of suitable monomers is found in U.S.
Pat. No. 4,076,663 at col. 2, lines 6-68 and col. 3, lines 1-12;
the teachings of which are hereby incorporated by reference.
In the event the monomer is an acid group containing monomer, the
monomer may be partially neutralized prior to polymerization or the
polymer neutralized subsequent to polymerization with an alkali
metal such as sodium or potassium or a compound such as ammonium
hydroxide. Additionally, the monomer may also be polymerized in the
presence of a preformed polymer to produce a graft polymer.
Exemplary preformed polymers are starch, polyvinyl alcohol,
carboxymethyl cellulose and other such polymers. Various methods
are described in the literature and are well known for the
preparation of superabsorbent polymers. Generally, these methods
involve some variant of an aqueous solution polymerization method
or an inverse suspension polymerization method. U.S. Pat. No.
5,145,906 illustrates the solution polymerization method and U.S.
Pat. No. 4,666,975 the inverse suspension method. Polymers made by
either method may be used in this invention.
The literature is replete with examples of attempts to improve upon
the performance of superabsorbent polymers in absorbent articles.
U.S. Pat. No. Re 32,649 claims that high gel volume, high gel
strength, low extractable content polymers give improved
performance. U.S. Pat. No. 5,147,343 describes an absorbent article
composed of a fiber matrix containing a high AUL (absorbency under
load) superabsorbent polymer having improved performance
properties; i.e. less leakage in diapers. This AUL performance
correlation is more fully described in the scientific literature,
see e.g. "The Concept of Superabsorbent Polymer" by Dr. F. Masuda,
Pira Fibrametrics Program, Paper 13 (Dec. 1987). Another example is
U.S. Pat. No. 5,145,906 which discloses improved performance using
a superabsorbent polymer having certain minimum polymer
properties.
A known technique for preparing superabsorbent polymers having
improved performance properties is to crosslink the polymer
particle's surface by a post crosslinking treatment. Crosslinking
the polymer chains on the surface of the superabsorbent particles
reduces the tendency of the particles to gel block and agglomerate
when wetted with aqueous liquid. Gel blocking is a reduction in
fluid absorption capacity caused by the formation of gel on the
polymer particle surface which blocks the transfer of fluid from
the polymer surface to its interior. This gel can also causes
polymer particles to agglomerate which further reduces fluid
absorption capacity. Various prior art patents describe the post
treatment of superabsorbent particles to surface crosslink the
polymer chains in the vicinity or near the particles' surface.
Illustrations of such teachings follow.
U.S. Pat. No. 4,666,983 describes surface crosslinking a
superabsorbent polymer using 0.001 to 10 parts by weight of a
difunctional crosslinking agent. The patent does not disclose using
any carrier solvent for the crosslinker. Surface crosslinking
agents are broadly disclosed such as polyhydric alcohols,
polyglycidyl ethers, polyfunctional amines and polyfunctional
isocyanates.
U.S. Pat. Nos. 4,507,438 & 4,541,871 disclose surface
crosslinking a superabsorbent polymer with a difunctional compound
(a crosslinker) in the presence of 0.01 to 1.3 parts by weight of
water and 0.1 to 50 parts by weight of an inert solvent per 100
parts of polymer. The crosslinker may be present in the amounts of
0.005 to 5% by weight. The patents discloses using a broad class of
crosslinking agents including glycidyl ethers, haloepoxies,
aldehydes and isocyanates with ethylene glycol diglycidyl ether
being the preferred crosslinker. A multitude of inert solvents are
described as useful in the invention. The solvents include
polyhydric alcohols with ethylene glycol, propylene glycol and
glycerine enumerated as preferred polyhydric alcohols. A mixed
solvent system is used to control the penetration of the
crosslinker into the particles interior. The use of more than 1.3
parts of water and less 0.1 parts of inert solvent per part
superabsorbent polymer are taught to be undesirable.
U.S. Pat. No. 5,140,076 discloses surface crosslinking using a
solvent comprising 0-50% water and 0-60% solvent. The reaction is
conducted in a lined, high speed mixer where the liner is required
to have a certain contact angle. The patent discloses the use of
polyhydric alcohol, diglycidyl ether, polyarizidene, urea, amine
and ionic crosslinkers.
U.S. Pat. No. 5,164,459 discloses a process for surface
crosslinking very similar to the above described '459 patent using
a polyhydric alcohol. crosslinker. The reaction is carded out
according to a specific equation to crosslink the resin's surface
and produce desired properties.
EPO 0 509,708 discloses surface crosslinking a superabsorbent
polymer with a polyhydroxy compound using a water based coating
solution which may contain a nonionic surfactant and optionally a
water soluble solvent. The process of the disclosure is said to
provide a superabsorbent having high absorption capacities, low
extractables and high gel toughness.
U.S. patent application Ser. No. 08/002346 filed Jan. 6, 1993
discloses a surface crosslinking process in which a mixed
water/solvent system is used. The solvent is selected from an
alkylene oxide of a monofunctional alcohol, a salt of an organic
acid and a lactam. A broad class of crosslinkers is disclosed
including polyamine - polyamide epichlorohydrin crosslinkers.
U.S. Pat. No. 4,666,975 describes an improved superabsorbent
polymer having a saline absorption quantity of 40-90 gms per gram
of polymer, an absorption rate of at least 8 ml per 0.3 grams of
polymer and a gel strength of 33 to 200 g/cm.sup.2. This
superabsorbent polymer is said to possess a crosslinking density
gradient in the polymer particles wherein the crosslinking density
is higher at the polymer particle surface than inside the
particle.
U.S. Pat. No. 5,002,986 discloses agglomerating and surface
crosslinking fine (<300 microns) superabsorbent particles to
provide superabsorbent polymer composition having a high absorbency
rate.
U.S. Pat. No. 5,229,466 discloses surface crosslinking
superabsorbent polymer using an aqueous solution of a
N-(hydroxyalkyl).beta.-(meth)-alanine ester and a water miscible
organic diluent.
Other patents which have described the surface crosslinking of
superabsorbent polymers are U.S. Pat. Nos.: 3,202,731; 4,043,952;
4,127,944; 4,159,260; 4,251,643; 4,272,514; 4,289,814; 4,295,987;
4,500,670; 4,587,308; 4,732,968; 4,735,987; 4,755,560; 4,755,562;
4,758,617; 4,771,105; 4,820,773; 4,824,901;4,954,562; 4,973,632;
4,985,518; 5,026,800.
This invention is directed to improved superabsorbent polymer
compositions, a process for their preparation, and absorbent
articles made from such compositions. The process of the invention
provides a superabsorbent polymer with superior fluid absorption
capacity, absorbency under pressure, high gel strength and low
extractables.
SUMMARY OF THE INVENTION
The invention is improved superabsorbent polymers having superior
fluid absorbency, a process for their preparation and absorbent
articles made therefrom. The superabsorbent polymers of the
invention have high fluid absorption capacity, high absorbency
under load, low gel blocking, low dust content and provide
absorbent articles having improved dryness and low leakage
properties.
The superabsorbent polymers of this invention have a 0.6 psi
absorbency under load (AUL) value of at least 20 g/g, preferably at
least about 25 g/g and a centrifuge retention (CRET) value of at
least about 35 gm/gm. These superabsorbent polymers exhibit
improved dryness and less leakage when incorporated into absorbent
articles such as diapers.
The improved superabsorbent polymers of the invention are prepared
from a base polymer which is a lightly crosslinked, partially
neutralized polymerization product of a carboxyl or carboxylate
group monomer or a carboxylic acid anhydride group monomer wherein
said polymerized monomer is present in an amount of from about 50
to about 99.5 mole percent. The preferred carboxyl group containing
monomer is acrylic acid.
The based polymer is lightly crosslinked as is evidenced by an
absorbency under load at 0.3 psi of 15 g/g or less, a centrifuge
retention (CRET) property of at least 35 g/g, preferably 40 g/g or
more, and a free polymer extractable content of less than 10
percent. According to the process of the invention, the base
polymer is surface crosslinked with an aqueous crosslinker solution
comprising water, a diol selected from a C.sub.3 to C.sub.6 diol
and a crosslinking compound. The water and diol components of the
crosslinker solution comprises from about 1.0 to about 6.0 percent
by weight based upon the weight of base polymer, preferably about
1.5 to about 5.5 percent and the crosslinker solution's surface
tension should be less than about 55 dynes per cm.
The crosslinking compound is selected from organic compounds which
contains two or more groups capable of reacting with the carboxy or
carboxylate groups of the polymer and is used in an amount of from
about 0.001 to about 3, preferably about 0.1 to about 1.0 percent
by weight based upon the weight of the polymer. Exemplary surface
crosslinkers are compounds containing epoxy, epichlorohydrin,
aziridinyl and azetidinium groups; preferably the crosslinker has a
molecular weight of at least 200.
The crosslinker solution is uniformly blended onto to the surface
of the superabsorbent particles and the mixture heated to crosslink
the polymer chains on or in the vicinity of the surface of the
superabsorbent particles.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The ability of a superabsorbent polymer to absorb aqueous fluids
generally decreases as the load or pressure on the polymer
increases. The superabsorbent polymers of this invention have a 0.6
psi absorbency under load (AUL) value of at least 20 g/g,
preferably 25 or higher and a centrifuge retention value of at
least about 35 g/g. Superabsorbent polymers having a 0.6 psi AUL
value of 20 g/g will exhibit an AUL at 0.3 psi of 30 g/g or more.
In a preferred embodiment of this invention, the superabsorbent
polymers will have a 0.3 psi AUL in excess of 30 g/g and a 0.6 psi
AUL value of 25 or more. These polymers exhibit improved dryness
and less leakage when incorporated into absorbent articles such as
diapers.
The superabsorbent polymers of this invention are prepared from
monoethylenically, unsaturated, water soluble carboxyl or
carboxylic acid anhydride containing monomers and the alkali metal
and ammonium salts thereof wherein said monomers comprise 50 to
99.9 mole percent of said polymer. Exemplary monomers include
acrylic acid, methacrylic acid, maleic acid, fumaric acid, maleic
anhydride and the sodium, potassium and ammonium salts thereof. The
preferred monomer is acrylic acid.
A base polymer is prepared from such water soluble monomers to
provide a lightly crosslinked, partially neutralized superabsorbent
polymer having a high aqueous fluid absorption capacity as
exemplified by high centrifuge retention properties. The definition
of superabsorbent polymer properties used herein and the applicable
test methods are set forth in the test method section of this
disclosure and the examples.
The base polymer useful in this invention is characterized by the
following properties:
______________________________________ Absorbency Under Load
.ltoreq. 15 g/g @ 0.3 psi Centrifuge Retention .gtoreq. 35 g/g
Extractables Content .ltoreq. 10% (wgt)
______________________________________
Monoethylenically, unsaturated monomers are polymerized in the
presence of an internal crosslinking compound to provide a lightly
crosslinked base polymer wherein the crosslinking is substantially
uniform throughout the polymer particles. These internal
crosslinkers are well known in the art. Suitable crosslinkers are
those compounds having two or more groups capable of reacting with
the monoethylenically unsaturated monomers and which are water
soluble or soluble in a water monomer mixture. The internal
crosslinking compound may be selected from a polyunsaturated
monomer such as divinylbenzene, a compound having at least two
functional groups which are reactive with the monoethylenically
unsaturated monomer such as ethylenediamine, a compound having at
least one unsaturated bond and at least one reactive functional
group such as glycidyl acrylate.
Exemplary internal crosslinkers are: tetraallyloxyethane,
N,N'-methylene bisacrylamide, N,N'-methylene bismethacrylamide,
triallylamine, trimethylol propane triacrylate, glycerol propoxy
triacrylate, divinylbenzene, N-methylol acrylamide,
N-methylolmethacrylamide, glycidyl methacrylate, polyethylene
polyamines, ethyl diamine, ethyl glycol, glycerine and the like.
Preferred internal crosslinking monomers are those containing at
least two allyl groups, most preferably four allyl groups. A
preferred internal crosslinker is tetraallyloxyethane. The amount
of internal crosslinker employed in the invention will depend on
the internal crosslinker and the polymerization method. Generally
the amount of internal crosslinker will vary from about 0.005% to
about 1.0 mole percent based on moles of ethylenically unsaturated
monomer.
Optional components used in the preparation of the superabsorbent
polymers of this invention are water soluble hydroxy group
containing polymers, such as polysaccharides and vinyl or acrylic
polymers. Examples of water soluble polysaccharides are starches,
water soluble celluloses and polygalactommans. Suitable starches
include the natural starches, such as sweet potato starch, potato
starch, wheat starch, corn starch, rice starch, tapioca starch and
the like. Processed or modified starches, such as dialdehyde
starch, alkyl-etherified starch, allyl-etherified starch,
oxylalkylated starch, aminoethyl-etherified starch, and
cyanomethyl-etherified starch are also suitable. Polyvinyl alcohol
and polyvinyl alcohol copolymers are also suitable.
The water-soluble celluloses useful in this invention are those
obtained from such sources as wood, stems, bast, seed fluffs and
the like which are then deriviatized to form hydroxyalkyl
cellulose, carboxymethyl cellulose, methyl cellulose and the
like.
Suitable polygalactomannans are guar gum and locust bean gums as
well as their hydroxyalkyl, carboxyalkyl, and aminoalkyl
derivatives. Water soluble vinyl and acrylic polymers include
polyvinyl alcohol and poly(hydroxyethyl acrylate). The preferred
polysaccharide for use in this invention is natural starch, such as
wheat starch, corn starch and alpha starches. These optional
preformed hydroxy containing polymers may be used in an amount from
about 1-15 percent, preferably about 1 to 10 percent, most
preferably about 1-5 percent.
The superabsorbent polymers in this invention may be prepared by
well known polymerization methods. The polymerization reaction is
conducted in the presence of redox initiators and thermal
initiators. The redox initiators can be used as the primary
initiator with the thermal polymerization initiators being used if
desired to reduce the free monomer content of the final
polymerization product below 0.1 percent by weight. Optionally,
thermal initiators or redox initiators may be used as the sole
initiator system. Examples of different initiator systems are found
in U.S. Pat. No. 4,497,930 which discloses a two component
initiator system comprising a persulfate and a hydroperoxide and
U.S. Pat. No. 5,145,906 which discloses a three component initiator
system; i.e. redox system plus thermal initiator.
Any of the well known water soluble reducing agents and oxidizing
agents can be used in this invention as the redox initiator.
Examples of reducing agents include such compounds as ascorbic
acid, alkali metal sulfites, alkali metal bisulfites, ammonium
sulfite, ammonium bisulfite, alkali metal hydrogen sulfite,
ammonium hydrogen sulfite, ferrous metal salts, e.g., ferrous
sulfates, sugars, aldehydes, primary and secondary alcohols, and
the like.
Oxidizing agents include such compounds as hydrogen peroxide,
alkali metal persulfate, ammonium persulfate, alkylhydroperoxides,
peresters, diacryl peroxides, silver salts, and the like. A
particularly preferred redox initiator pair is ascorbic acid and
hydrogen peroxide. The reducing agent is used in an amount of about
2.times.10.sup.-5 to about 2.0.times.10.sup.-2 mole percent based
on moles of acrylic acid. The amount of oxidizing agent used will
vary from about 2.0.times.10.sup.-3 to about 1.1 mole percent,
based on moles of acrylic acid.
In order to ensure complete polymerization of the unsaturated
monomer and the crosslinking monomer, a thermal initiator can be
included in the polymerization process. Useful thermal initiators
am the "azo" initiators, i.e. compounds which contain the
--N.dbd.N-- structure. Any of the azo compounds which have
solubility in water or in a monomer-water mixture and which have a
10 hour half life at 30.degree. C. or above can be used. Examples
of useful azo initiators are 2,2'-azobis(amidino) propane
dihydrochloride, 4,4'-azobis (cyanovaleric acid),
4,4'-butylazo-cyanovaleric acid, 2,2'-azobis(isobutyronitrile ),
and the like. Other thermal initiators include the persulfates and
hydroperoxides when used in the absence of a reducing agent e.g.,
sodium, potassium and ammonium persulfates, t-butylhydroperoxide
and the like. A preferred azo initiator for use in this invention
is 2,2'-azobis(amidinopropane) dihydrochloride. The thermal
initiators are used in the amount of 0 to about 1 mole percent
based on the weight of unsaturated monomer.
The base polymer may be prepared by the solution or the inverse
suspension polymerization method or any suitable bulk
polymerization method. Preferably, the base polymer is prepared
according to the solution polymerization method. The solution
polymerization and inverse polymerization methods are well known in
the art; see for example U.S. Pat. Nos. 4,076,663; 4,286,082;
4,654,039 and 5,145,906 which describe the solution polymerization
method and U.S. Pat. Nos. 4,340,706; 4,497,930; 4,666,975;
4,507,438 and 4,683,274 which describe the inverse suspension
method. The teachings of these patents are hereby incorporated by
reference.
In the solution polymerization method, the water soluble monomer is
polymerized at a monomer concentration from about 5 to about 30
percent in aqueous solution at a temperature from about 5.degree.
C. to about 150.degree. C. depending upon the polymerization
initiator system. A detailed description of the solution
polymerization method is given in U.S. Pat. No. 5,145,906; the
teachings of which are hereby incorporated by reference.
In the inverse suspension polymerization process, the unsaturated
monomer in an aqueous solution (about 35 to 60 percent monomer to
65 to 40 percent water) is dispersed in an alicyclic or aliphatic
hydrocarbon suspension medium in the presence of a dispersing agent
such as a surfactant or protective colloid such as polyvinyl
alcohol. A surfactant having a HLB value of 8-12 such as a sorbitan
fatty acid ester ether may employed as the dispersing agent. The
inverse suspension polymerization method is described in detail in
U.S. Pat. No. 4,340,706; the teachings of which are hereby
incorporated by reference.
The carboxylic acid groups of the unsaturated monomer used in the
polymerization have to be partially neutralized. Suitable
neutralizing agents include an alkali such as sodium hydroxide,
ammonium hydroxide, potassium hydroxide or the like, and the
appropriate degree of neutralization is 50-98 mole percent;
preferably 60-75 percent. The degree of neutralization should be at
least 50 mole percent. Low neutralization levels (less than 50 mole
percent) give superabsorbent polymers having low absorbency
properties.
The base polymer is prepared by either the solution or inverse
polymerization method dried and screened to provide a
superabsorbent particle with an appropriate particle size
distribution and particle shape. Generally the superabsorbent
particle size distribution should be between 100 and 850 microns,
preferably between 150 and 600 microns. Large particles over 850
microns are undesired since they do not always feed well in
machinery used to make absorbent articles, they tend to cause the
absorbent article to have an abrasive feel and they do not function
well from an absorption efficiency standpoint.
Small fine particles below 100 microns are not desired because they
gel block. Particles below 10 microns are not desired because they
dust and present an industrial hygiene problem. Surprisingly the
products of this invention have a low tendency to dust as evidenced
by their low airborne particle count. Airborne particles are not
desired from an industrial hygiene standpoint because they can be
inhaled or respirated by workers in manufacturing operations and
caused lung irritation.
The base superabsorbent polymer is treated with a crosslinker
solution containing from about 0.5 to about 3.5 weight percent
water, from about 1.0 to 2.5 weight percent of a water miscible
solvent selected from a C.sub.3 to C.sub.6 diol and a crosslinker
having at least two functional groups that can react with the
carboxyl, carboxylate or other reactive groups in the
superabsorbent polymer chain to crosslink the polymer chains on or
in the vicinity of the surface of the superabsorbent polymer
particles. The term "diol" is intended to mean a dihydroxy
aliphatic compound which may be a linear or branched compound, (a
glycol). The term "surface crosslinking" used in this description
and the claims hereof is used to describe this process of
crosslinking the polymer chains on or in the vicinity of the
particle's surface. The terms, "surface crosslinker" and "surface
crosslinker solution" are likewise used to describe the
crosslinking compound and the solution used to effect this surface
crosslinking process. The crosslinking compound is used in an
amount of from about 0.01 to about 3 weight percent, preferably
0.1% to 1.5% and most preferably about 0.25% to 1% based upon the
weight of the superabsorbent polymer. The surface crosslinker may
be selected from diglycidyl ethers, haloepoxy, isocyanate,
aziridinyl, azetidinium group containing compounds, polyamine,
polyamine-polyamide, polyamine-epichlorohydrin adducts and
amine-polymer-epichlorohydrin adducts and the like. Preferred
crosslinkers are the higher molecular weight diglycidyl ether
compounds of at least 200 and the polymeric-epichlorohydrin adducts
having a molecular weight average in excess of 2000.
Exemplary surface crosslinkers are poly (ethylene glycol)
diglycidyl ethers, poly (propylene glycol) diglycidyl ethers,
epichlorohydrin substituted compounds, methyl-epichlorohydrin
substituted compounds, hexamethylene diisocyanate, triethylene
triamine, polyethylene amine,
2,2-bishydroxymethylbutanol-tris[3-(1-azindinyl)propionate],
polyamine epichlorohydrin adducts,
polyethylene-polyamine-epichlorohydrin adducts, and the like.
The preferred surface crosslinkers are the higher molecular weight
diglycidyl ether compounds, polyamide (polyamide-polyamine)
epichlorohydrin adducts, polyamine epichlorohydrin adducts and
amine polymer epichlorohydrin adducts. Polyamide-epichlorohydrin
adducts are prepared by reacting epichlorohydrin with the
polycondensation product of a polyalkylene polyamine with a
polycarboxylic acid such as diethylene triamine with a dibasic acid
such as adipic acid. Polyamine epichlorohydrin adducts are made by
condensing a polyalkylene polyamine directly with epichlorohydrin.
These adducts include polyalkylene polyamines which are linked
together with dihalides to form higher polyamines before reacting
them with epichlorohydrin. Amine polymer epichlorohydrin adducts
include resins in which the monomeric amine is polymerized to a
polyamine precursor which is then alkylated and reacted with
epichlorohydrin. They include amines substituted polymers of vinyl,
allyl, acrylate or epoxy monomers. The epichlorohydrin adducts
whether the polymer is a polyamide, a polyamine or an amine polymer
react with the epichlorohydrin by different routes. If the amino
group in the polymer chain is a primary amine, two epichlorohydrin
molecules reacted with the nitrogen and form a disubstituted
chlorohydroxypropyl substituted amine group. Secondary amine groups
react with epichlorohydrin to form a tertiary aminochlorohydrin
group which cyclizes to form a reactive 3-hydroxyazetidinium salt
moiety. This is a preferred reactive group. Tertiary amine groups
react with epichlorohydrin to form a glycidyl; (2,3 epoxypropyl)
ammonium salt. Preferably the reactive group is an azetidinium
moiety. However, these adducts may contain a mixture of
chlorohydroxypropyl, epoxy and azetidinium groups. Preferably the
epichlorohydrin adducts have a molecular weight of at least 2,000;
preferably 300,000 to 500,000 and wherein at least 50 mole percent
of the reactive groups in the adduct are the azetidinium group. A
preferred polymer is one in which about 90% of the substitution is
in the form of an azetidinium group and about 10% as an epoxide
group. Exemplary products are Reten.RTM. 204LS and Kymeme.RTM. 736
epichlorohydrin adducts; available from Hercules Inc., Wilmington,
Del. These products are sold as an aqueous solution of the reactive
epichlorohydrin adduct. The Reten.RTM. 204LS product is available
as a 15% aqueous solution and the Kymeme.RTM. 736 product as a 38%
aqueous solution.
The surface crosslinker solution should have a surface tension not
greater than about 55 dynes per cm; preferably not greater than
about 50 dynes per cm; e.g. about 40 to about 50 dynes per cm. In
the event the surface tension of the crosslinker solution is higher
than about 55 dynes per cm; the surface crosslinked polymer has
inferior absorbency as evidenced by a low 0.6 psi AUL value. While
not being bound to any theory, it is believed that when the surface
tension of the crosslinker solution is higher than about 55 dynes
per cm, the solution is not uniformly distributed on the surface of
the polymer particles and a lower absorbency value results.
Optionally, a surfactant may be used to reduce the surface tension
of the crosslinker solution.
The desired surface tension is achieved by adding the C.sub.3 to
C.sub.6 dihydroxy compound to water component of the crosslinker
solution to achieve a surface tension below about 55 dynes/cm
range. The amount of each solvent is determined by simple
experimentation. Generally the crosslinker has a negligible effect
on the surface tension of the crosslinker solution. The diols
useful in the invention are propylene glycol, butylene glycol,
pentanediol and hexanediol. Ethylene glycol was found to be an
undesired solvent because it tends to swell the superabsorbent
polymer particles and their surfaces becomes tacky which results in
undesired particle agglomeration. In addition, ethylene glycol is
undesirable because of its toxicity and biodegradability
properties. The C.sub.3 to C.sub.6 diol is used in an amount of
from about 1 percent by weight to about 2.5 percent by weight based
upon the weight of superabsorbent polymer; preferably about 1 to
about 2 percent by weight. The water component of the crosslinker
solution comprises about 0.5 to 3.5 percent by water based upon the
weight of the polymer, preferably about 1.5 to 2.0 percent.
The total amount of crosslinker solution used depends upon the type
of equipment and the method used to coat the base polymer with the
surface crosslinking solution. Generally the amount of crosslinker
solution should be about 1.5% minimum based on the weight of the
polymer. The crosslinker solution is applied to the base polymer
particles in a manner such that the solution is uniformly
distributed on the surface of the base polymer particle. Any of the
known methods for dispersing a liquid can be used; preferably by
dispersing the crosslinker solution into fine droplets; e.g. by use
of a pressurized nozzle or a rotating disc. Uniform crosslinker
dispersion on the base polymer can be achieved in a high intensity
mechanical mixer or a fluidized mixture which suspends the base
polymer in a turbulent gas stream. Methods for the dispersion of a
liquid onto the superabsorbent base polymer's surface are known in
the art; see for example U.S. Pat. No. 4,734,478; the teachings of
which are hereby incorporated by reference; in particular column 6,
line 45 to column 7, line 35.
Exemplary commercially available equipment for conducting the
crosslinker solution dispersion step of the invention are high
speed variable intensity paddle mixers such as the "Turbulizer"
mixer of the Bepex Corporation, Rolling Meadows, Ill. or the high
speed variable intensity vertical mixer sold by Bepex under the
tradename, "Turboflex". These machines are generally operated in a
continuous manner using a short residence time in the order of 2
seconds to 2 minutes, typically 2-30 seconds. Dispersion may be
effected batchwise in a high intensity mixer such as a Henschel
mixer or in liquid-solid V-blender equipped with a liquid
dispersion device. In any event, whether a batchwise or continuous
dispersion method is used, simple experimentation can be conducted
to determine the best process conditions for the particular machine
employed in the process. Preferably, the surface crosslinker is
coated onto the polymer particles under high intensity mixing
conditions.
After effecting dispersion of the surface crosslinker on the base
polymer particle the crosslinking reaction is effected and the
polymer particle dried. The crosslinking reaction may be effected
at a temperature from about 70.degree. C. to about 150.degree.
C.
TEST METHODS
The following test methods were used to determine the properties of
the superabsorbent polymers described herein.
Absorbency Under Load (AUL) at 0.3 psi
This test is designed to determine the absorbency under load of a
superabsorbent material. The amount of saline (0.9% wt/% NaCl
aqueous solution) absorbed with the weight applied to the polymer
indicates the effectiveness of the polymer's absorbency in a diaper
system under the weight of a baby.
Absorbency under load is measured using a plastic petri dish with
elevating rods and a 1.241""OD.times.0.998""ID.times.1.316"" long
plexiglass tube with a wire net (100 mesh) at the bottom of the
tube. The particle size of the test samples is between 30 to 50
mesh, (through 30 and retained on 50).
A test sample, 0.160.+-.0.01 g is weighed out and recorded as
S.sub.1. The sample is placed in the plastic tube and is spread
evenly over the wire net. A 100 g weight and a disc are placed on
the sample. The assembly (polymer sample, tube, disc and weight) is
weighed and recorded as W.sub.1. The assembly is then placed in a
petri dish containing 40 ml 0.9% saline aqueous solution. After one
hour of absorption, the assembly is removed from petri dish and
excess saline blotted from the bottom. The assembly is weighed
again and this value recorded as W.sub.2. Absorbency under load
(AUL) is equal to (W.sub.2 -W.sub.1)/S.sub.1 and is express in
g/g.
Absorbency Under Load (AUL) at 0.6 psi
The absorbency under load at 0.6 psi is determined in the same
manner as the above described absorbency under load at 0.3 psi
except a 200 gram weight is used instead of the 100 gram
weight.
Surface Tension
An adequate amount of liquid is transferred into sample holding cup
(a Fisher Surface Tensiomat Model 21) and then the sample cup is
placed onto the sample table of the tensiometer. A clean
platinum-iridium ring is placed in the calibrated tensiometer and
the sample table is raised until the platinum-iridium ring is below
the surface of the liquid. After the ring has been submerged in
fluid for about 30 seconds, the torsion arm is released so it hangs
freely and the reference arm is adjusted to be parallel with the
line on the mirror of the tensiometer. The ring is slowly raised at
a constant rate. The reference arm is maintained parallel to the
line on the mirror by lowering the sample cup when necessary. When
the ring breaks free from the fluid surface, the number on the
front dial is of the tensiometer is recorded. This is the surface
tension in dynes/cm. The measurement is conducted at
.about.23.degree. C., repeated three times and the average value
reported.
Centrifuge Retention Capacity (CRET)
This test is designed to measure the amount of saline solution
retained inside a superabsorbent polymer when under a specific
centrifuge force.
Approximately 0.200 grams of superabsorbent polymer are placed into
a sealable tea bag (7.5 cm.times.6.5 cm) and the tea bag sealed.
The tea bag and polymer are immersed in a 0.9% saline solution for
30 minutes and then centrifuged for three minutes at 1600 rpm on a
21.6 cm diameter centrifuge. The weight difference before
centrifuging and after is the amount of saline solution absorbed by
polymer gel which is divided by original dry polymer weight and
this value is the centrifuge retention capacity of the polymer
expressed in g/g.
The following examples illustrate the preparation of the
superabsorbent polymers of the invention and their preparation.
These examples are intended to be illustrative and are not intended
to limit the scope of the invention or the claims. In this
description (unless otherwise specified) percent values are weight
percent and molecular weight of a polymeric compound is weight
average molecular weight. Surface tension measurements and values
are made and reported at 23.degree. C.
EXAMPLE 1
This example illustrates the preparation of a base polymer having a
0.3 psi AUL of 15 g/g or less and centrifuge retention value of 40
g/g.
Into a 4-liter reaction vessel under a nitrogen atmosphere are
added 816 grams of acrylic acid, 306 grams of an eight-percent
starch solution, 4.5 grams of tetraallyloxyethane and 2840.2 grams
of demineralized water. The mixture is sparged with nitrogen and
cooled to 10.degree. C. and 16.32 grams of 0.1% aqueous hydrogen
peroxide solution, 12.24 grams of 0.1% aqueous ascorbic acid
solution and 4.76 grams of 10% aqueous 2,2-azobis(amidino)propane
dihydrochloride solution are added to the reaction vessel.
The polymerization reaction is initiated and the reaction
temperature increases to 60.degree. C. from the heat of the
reaction. The reaction is continued for 6 hours at 60.degree. C.
The reaction product is a gel which is chopped into pieces. The
chopped reaction gel is neutralized to about 70 mole percent with
656.6 grams of 50 percent aqueous sodium hydroxide. The gel is
chopped three more times to uniformly mix and neutralized the gel.
The gel is then dried on a rotary drum dryer at 150.degree. C. to
200.degree. C. and the resulting superabsorbent flake is ground
into a powder with the following polymer properties and particle
size distribution.
______________________________________ Polymer Properties 0.3 psi
AUL 13.0 g/g 0.6 psi AUL 6.0 g/g CRET 46.2 g/g Particle Size
Distribution U.S. Std. Mesh Wgt. Percent 20 0.1 30 12.2 50 44.6 170
41.7 325 1.0 -325 0.3 ______________________________________
EXAMPLE 2
A laboratory liquid--solid V-blender, fitted with a liquid
dispersion bar (Patterson-Kelley Corp., East Stroudsburg, Pa.,
Model number C134200), was charged with 1.25 kg of the base polymer
from Example 1. The blender was started to begin mixing, the liquid
dispersion bar was engaged and a mixture comprised of 21.9 g of
propylene glycol (Olin Chemical, Stamford, Conn.) and 3.12 g
(0.25%) of an amine polymer-epichlorohydrin adduct as a 15%
solution in water (Reten.RTM. 204LS, Hercules Incorporated,
Wilmington, Del.) was added to the blender over a one minute
period. The weight of the crosslinker solution (propylene glycol,
water and amine polymer-epichlorohydrin adduct) was 4.25% based on
the weight of polymer and the solution had a surface tension value
of 50.7 dynes/cm.
After completion of the crosslinker solution addition, mixing was
continued for 2 minutes. The polymer was then heated to 150.degree.
C. and held at 150.degree. C. for 90 minutes to effect the surface
crosslinking reaction and dry the polymer.
The product had the following properties and particle size
distribution:
______________________________________ Polymer Properties 0.3 psi
AUL 31.4 0.6 psi AUL 26.9 CRET 35.8
______________________________________ Particle Size Distribution
U.S. Std. Mesh Wgt. Percent ______________________________________
20 0.14 30 31.5 50 52.4 170 15.8 325 0.2 -325 0.0
______________________________________
EXAMPLE 3
The procedure used in this example is substantially identical to
that used in Example 2, but the crosslinker concentration was
increased to 0.375% (4.69 grams of amine polymer-epichlorohydrin
adduct) and 2.125% water was added to maintain the crosslinker
solution concentration the same as that of Example 2. The surface
tension of the crosslinker of the solution was 49.5 dynes/cm. The
product had the following properties:
______________________________________ 0.3 psi AUL 32.9 0.6 psi AUL
22.0 CRET 38.1 ______________________________________
EXAMPLE 4
The procedure used in this example is substantially identical to
that used in Example 2, but the crosslinker concentration was
increased 0.5% (amine polymer-epichlorohydrin adduct) and 2.0%
water was added to maintain the amount of the crosslinker solution;
the same as that of Example 2. The crosslinker solution's surface
tension was 50.1 dynes/cm. The product had the following
properties:
______________________________________ 0.3 psi AUL 31.5 0.6 psi AUL
26.3 CRET 35.7 ______________________________________
EXAMPLE 5
The procedure used in this example is substantially identical to
that used in Example 2, but the crosslinker solution contained
0.97% crosslinker (amine polymer-epichlorohydrin adduct) and 1.59%
water was added to maintain the crosslinker solution concentration
the same as that of Example 2. The crosslinker solution's surface
tension was 48.8 dynes/cm. The product had the following
properties:
______________________________________ 0.3 psi AUL 32.4 0.6 psi AUL
25.5 CRET 36.8 ______________________________________
The following Table 1 summarizes the data of examples 1-5. In all
examples, the amount of crosslinker solution was 4.25 weight
percent based on the weight of polymer and the diol was propylene
glycol.
TABLE 1 ______________________________________ (Examples 1-5) % %
Ex. XL PG % Water .sigma. 0.3 AUL 0.6 AUL CRET
______________________________________ 1 0 0 0 0 13 6 46.2 2 0.25
1.75 2.25 50.7 31.4 26.9 35.8 3 0.375 1.75 2.125 49.5 32.9 22.0
38.1 4 0.50 1.75 2.0 50.1 31.5 26.3 35.7 5 0.97 1.69 1.59 48.8 32.4
25.5 36.8 ______________________________________ % XL = %
crosslinker, % PG = % propylene glycol, .sigma. = surface tensio
dynes/cm, AUL = absorbency under load g/g, CRET = centrifuge
retention g/g
EXAMPLE 6
This example is substantially identical to Example 3, but 21.9 g of
ethylene glycol (Aldrich Chemical, Milwaukee, Wis.), was used
instead of propylene glycol. The crosslinker solution's surface
tension was 61.6. The product had the following properties:
______________________________________ 0.3 psi AUL 31.5 0.6 psi AUL
24.8 CRET 36.8 ______________________________________
Although the polymer absorbency properties were good; the ethylene
glycol caused the polymer particles to swell and particles
agglomerated into undesired large particles.
EXAMPLE 7
The procedure used in this example is substantially identical to
that used in Example 3, but 21.9 g of 1,3-butanediol (Aldrich
Chemical, Milwaukee, Wis.), was used instead of propylene glycol.
The crosslinker solution's surface tension was 50.1 dynes/cm. The
product had the following properties:
______________________________________ 0.3 psi AUL 32.8 0.6 psi AUL
23.5 CRET 38.8 ______________________________________
EXAMPLE 8
This example is substantially identical to Example 3, but 21.9 g of
1,5-pentandiol (Aldrich Chemical, Milwaukee, Wis.), was used
instead of propylene glycol. The crosslinker solution's surface
tension was 43. The product had the following properties:
______________________________________ 0.3 psi AUL 31.7 0.6 psi AUL
20.7 CRET 37.9 ______________________________________
The following Table 2 summarizes the data of examples 6, 7 & 8.
The data of Example 3 is also included for reference. These
examples had the following common factor: crosslinker solution
-4.25%; crosslinker concentration -0.375%; diol concentration
-1.75%; and water concentration -2.125%. All percentages are weight
percent based on the weight of polymer.
TABLE 2 ______________________________________ (Examples 3, 6-8)
Ex. Solvent Surface Tension 0.3 AUL 0.6 AUL CRET
______________________________________ 3 PG 49.5 32.9 22.0 38.1 6*
EG 61.6 31.5 24.8 36.8 7 BD 50.1 32.8 23.5 38.8 8 PD 43 31.7 20.7
37.9 ______________________________________ * = excessive particle
agglomeration observed, PG = propylene glycol, EG ethylene glycol,
BD = butanediol, PD = pentanediol
EXAMPLE 9
The procedure used in this example is substantially identical to
that used in Example 3, but the crosslinker solution contained
40.63 g propylene glycol (3.25%); 4.75 g (0.38%) of amine
polymer-epichlorohydrin adduct (Kymeme.RTM. 736; Hercules
Incorporated, Wilmington, De.; 38% solution in water), 7.75 g
(0.62%) water. The amount of crosslinker solution was 4.25 weight
percent based on polymer and it had a surface tension was 42.7. The
product had the following properties:
______________________________________ 0.3 psi AUL 33.2 0.6 psi AUL
20.6 CRET 38.8 ______________________________________
EXAMPLE 10
The procedure for this example is substantially identical to that
used in Example 9, but the amount of propylene glycol was decreased
to 18.8 g (1.5%). The crosslinker solution was 2.5 weight percent
based on polymer and it had a surface tension of 44.3. The product
had the following properties:
______________________________________ 0.3 psi AUL 33.2 0.6 psi AUL
28.2 CRET 36.0 ______________________________________
EXAMPLE 11
The procedure for this example is substantially identical to that
used in Example 9, but the propylene glycol amount was decreased to
12.5 g (1.0%). The crosslinker solution was 2.0 weight percent
based on polymer and it had a surface tension of 46.2. The product
had the following properties:
______________________________________ 0.3 psi AUL 34.6 0.6 psi AUL
28.9 CRET 37.6 ______________________________________
EXAMPLE 12
The procedure for this example was substantially identical to that
used in Example 9, but the amount of propylene glycol was reduced
to 6.25 g (0.5%). The amount of crosslinker solution was 1.5
percent by weight of polymer and it had surface tension of 50.2.
The product had the following properties:
______________________________________ 0.3 psi AUL 34 0.6 psi AUL
16 CRET 41 ______________________________________
Table 3 summarizes the data of Examples 9-12. In these examples,
the amount of crosslinker and water was held constant at 0.38% and
0.62% respectively. The amount of propylene glycol was
progressively reduced. At a propylene glycol level less than 1.0
weight, the 0.6 psi decreased to 16 g/g.
TABLE 3 ______________________________________ (Examples 9-12) %
Ex. PG % XL Solution .sigma. 0.3 AUL 0.6 AUL CRET
______________________________________ 9 3.25 4.25 42.7 33.2 20.6
38.8 10 1.5 2.5 44.3 33.2 28.2 36.0 11 1.0 2.0 46.2 34.6 28.9 37.6
12 0.5 1.5 50.2 34.0 16.0 41.0
______________________________________ % PG % propylene glycol, %
XL = % crosslinker solution, .sigma. = surfac tension dynes/cm, AUL
= absorbency under load g/g, CRET = centrifuge retention g/g
EXAMPLES 13-15
The procedure of Example 3 was repeated except the propylene glycol
was progressively reduced from 1.0% to 0.5% to 0% while the
crosslinking solution amount was held at 4.25%. Table 4 summarizes
the results of these experiments.
TABLE 4 ______________________________________ (Examples 13-15) Ex.
% PG % Water .sigma. 0.3 AUL 0.6 AUL CRET
______________________________________ 13 1.0 2.88 56.9 32.9 14
39.3 14 0.5 2.38 63.3 12.4 8.5 43.2 15 0.0 3.88 74.7 9.4 8.0 44.8
______________________________________ % PG = % propylene glycol,
.sigma. = surface tension dynes/cm, AUL absorbency under load g/g,
CRET = centrifuge retention g/g
These examples illustrate the necessity to maintain the surface
tension of the crosslinker solution below about 55 dynes/cm.
EXAMPLES 16-18
The procedure for Example 3 was substantially repeat except
butanediol (butylene glycol) was used instead of propylene glycol.
The amount of butylene glycol was progressively reduced from 1.5%
to 1.0% to 0.5%. Table 5 summarizes the results of these
experiments. The amount of crosslinker solution was maintained at
4.25% and the crosslinker at 0.375%.
TABLE 5 ______________________________________ (Examples 16-18) Ex.
% BG % Water .sigma. 0.3 AUL 0.6 AUL CRET
______________________________________ 16 1.5 2.38 51.4 32.9 26.7
36.5 17 1.0 2.88 55.1 33.3 26.3 37.8 18 0.5 3.38 60.8 32.8 18.4
38.5 ______________________________________ % PG butylene glycol,
.sigma. = surface tension dynes/cm, AUL = absorbency under load
g/g, CRET = centrifuge retention g/g
EXAMPLE 19
The procedure of Example 3 was substantially repeated except the
crosslinker was diethylene glycol diglycidyl ether (MW 224) and the
weight percent of the crosslinker solution was 3.95% and the amount
of crosslinker was 0.33%. The crosslinker solution's surface
tension was 49.8. The product had the following properties:
______________________________________ 0.3 psi AUL 34 0.6 psi AUL
28.8 CRET 33.8 ______________________________________
EXAMPLE 20
The procedure of Example 3 was substantially repeated except the
propylene glycol was replaced with an equal amount of glycerine.
The crosslinker solution had a surface tension of 70.2 dynes per
centimeter. The resulting polymer had the following properties:
______________________________________ 0.6 psi AUL 11.2 0.3 psi AUL
19.1 CRET 41.2 ______________________________________
EXAMPLE 21
The products of Example 1 and Example 2 were analyzed to determine
the dust content of each product. Twenty grams of product was
slowly added to a 500 ml vacuum flask via a funnel having a 150 mm
diameter and a 230 mm height. The vacuum port of the flask was
connected to a Model P-5 Digital Dust Analyzer manufactured by
Sibata Scientific Technology Ltd., Tokyo, Japan, and distributed by
MDA Instruments, Lincolnshire, IL. The dust analyzer was calibrated
for background atmospheric dust levels in the standard manner prior
to addition of the test sample. The dust content of each sample was
measured at one minute after completion of the sample addition to
the flask. An average of three tests is reported below for each
sample in dust counts per minute (CPM).
______________________________________ Dust Content
______________________________________ Example 1 13.3 .+-. 3.2 cpm
Example 2 1.7 .+-. 0.6 cpm
______________________________________
Additionally it has been found that a C.sub.3 -C.sub.6 diol can be
used to reduce the dust content of superabsorbent polymer
compositions. Optionally the diol may be applied as an aqueous
solution. The amount of diol employed depends, in part, upon the
equipment used to effect dispersion of the diol on the
superabsorbent polymers and the amount of dust present in the
composition.
Generally, the effective amount of diol will be between 0.1 to
about 1 weight percent; preferably about 0.3 to about 0.6 weight
percent. An equal amount of water may be used to facilitate the
dispersion. However, the addition of water may be necessitate
drying the polymer to remove undesired moisture.
EXAMPLE 22
This example illustrates treating a superabsorbent polymer
composition with an aqueous solution of propylene glycol in an
amount of about 1% of glycol and about 1% of water.
1750 parts of SANWET.RTM. IM-3900 water absorbing resin (Lot
#32204) were placed into a Patterson-Kelley laboratory-size
V-Blender. An organic aqueous solution, 17.5 parts of propylene
glycol and 17.5 parts of D.I. water, was sprayed onto the
superabsorbent polymer powder and uniformly mixed. The resulting
mixture was heated for about 30 minutes at 130.degree. C.
Airborne dust content of the resulting superabsorbent polymer was
determined by Digital Dust Indicator, Model P-5, manufactured by
Sibata Scientific Technology, Ltd. and is reported in dust counts
per minute.
Absorption properties of the resulting superabsorbent polymer were
analyzed by absorbency under load (AUL) method at 0.3 psi.
Comparative results are listed as follows:
______________________________________ Polymer Dust Count AUL (g/g)
______________________________________ Before 157 .+-. 20 25.7 .+-.
1.0 After 3 .+-. 1 28.3 .+-. 1.6
______________________________________
EXAMPLE 23
The procedure of Example 22 is substantially repeated except the
amount of propylene glycol was reduced to 0.4% based upon the
weight of the polymer and the water concentration was reduced to
zero. A similar low dust content (4 cpm) is obtained on the treated
polymer.
The superabsorbent polymers of this invention are useful in the
manufacture of moisture absorbent articles, such as disposable
diapers, sanitary napkins, incontinence garments, bandages, and the
like. The superabsorbent compositions of this invention are
particularly useful in the manufacture of thin and ultra thin
disposable diapers which have excellent moisture absorbance
capacity, fluid distribution properties and reduced leakage.
In making absorbent articles with the compositions of this
invention, the superabsorbent composition may be mixed with,
attached to, layered in, or dispersed in a porous matrix of fibers.
Such matrices are made with hydrophilic fibers such as wood pulp or
fluff, cotton liners, and synthetic fibers or a mixture of the
fibers and the wood fluff. The fibers can be loose or joined as in
nonwovens. The synthetic fibers can be polyethylene, polypropylene,
polyesters, copolymers of polyesters and polyamides and the like.
The synthetic fibers may be meltblown fibers or fibers which have
been treated to render them hydrophilic. Additionally, the
superabsorbent polymers of the invention may be incorporated in the
absorbent article in a compartment or localized area of the
absorbent structure.
Absorbent articles, such as disposable diapers, are made with a
liquid-impermeable backing material, a liquid-permeable bodyside
facing material and the liquid-absorbing composite sandwiched
between the backing material and the facing material. The
liquid-impermeable backing material can be made from commercially
available polyolefin film and the liquid-permeable facing material
can be made from a commercially available nonwoven material, such
as spunbonded or corded fibrous web which is wettable and capable
of passing urine.
The absorbent articles of the invention may comprise from about 5%
to about 90% by weight of the superabsorbent polymers of the
invention. In a typical absorbent article, the superabsorbent
polymer of the invention may be dispersed in a fiber matrix in
which the superabsorbent is present in an amount from about 30 to
70 weight percent and the fiber matrix comprising 70 to 30 weight
percent of the article. In another form of absorbent article, the
superabsorbent may be present in a containment structure in which
the superabsorbent polymer is present in an amount of about 30 to
90 percent by weight. Combinations of dispersed superabsorbent
polymer and contained superabsorbent polymer are also known.
The superabsorbent polymers of this invention can be used in the
manufacture of absorbent articles such as those described in U.S.
Pat. Nos. 3,669,103; 3,670,731; 4,654,039; 4,699,823; 4,430,086;
4,973,325; 4,892,598; 4,798,603; 4,500,315; 4,596,567; 4,676,784;
4,938,756; 4,537,590; 4,935,022; 4,673,402; 5,061,259; 5,147,343;
5,149,335; and 5,156,902; the teachings of which are hereby
incorporated by reference.
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